One of the issues is that there is more than one way of marking a bike. The British Transport police put a chip down the seatpost of my bike and the Met used some sort of code pasted onto the frame. Each thinks their system is the best (perhaps they aren't even aware of what the other is doing). Add that to whatever systems police forces outside London are using and it's easy to identify that marking is pretty hopeless. Would a Met police officer find a chip/tag in the seat tube of my bike? I doubt it very much. Would a BTP officer look for a code on the outside of my bike? Guess not. Would either of them be able to find the bike's own serial code and bother to search any of the 3 databases that it's on....I don't think so.
My experience is that the Met lost-and-found process is awful. I handed a bike in, fully aware that if it was unclaimed that I could claim it. After the period had elapsed I went to collect the bike. They told me that it had been either 'given to a charity' or 'sold' because they couldn't store bikes for very long (about two weeks). When I complained in writing, they had a look in their yard and found the bike. Therefore they actually had no idea whether they had the bike or not, but thought it was quicker to tell me they didn't have it. If they don't know what they've got they can't be that eager to return a bike to its owner. Despite the truly awful return rate, the police do like to code bikes because it makes it look as if they are being pro-active when actually it's just easy and ineffective.